The use of magnetometers, particularly of induction loops, for the contactless detection of vehicles is known from the prior art. Since the installation of induction loops is space-consuming and complex, there is an intensive search for alternative concepts that use compact magnetometers, in particular magnetoresistive sensors. Attempts to date have hitherto had only limited success. The reason for this is primarily that the measurement signal from compact magnetometers is proportional to the magnetic field strength at a physical point, i.e. the position of the actual sensor element. This means that the measurement signal from compact magnetometers cannot be used to distinguish whether a vehicle is situated immediately above or below the magnetometer or just in proximity thereto. The same applies to moving vehicles, i.e. it is not possible to use the measurement signal from a compact magnetometer to establish whether the vehicle is being moved past the magnetometer immediately above or below the magnetometer or with a lateral or horizontal offset of up to 20 meters. Accordingly, a compact magnetometer cannot be used to distinguish vehicle types, such as car, truck, etc., from one another with sufficient certainty in stationary and flowing traffic. In addition, the apparatuses based on compact magnetometers are susceptible to failure and have an error rate of greater than 3%.
Within the context of the present invention, the term “error rate” denotes any error that occurs during the sensing and classification of stationary or moving vehicles, particularly of vehicles moving in flowing traffic. Furthermore, the term “vehicle” within the context of the present invention covers various vehicle classes, particularly cars, cars with a trailer, trucks, trucks with a trailer, vans, buses, articulated trucks and motorcycles.
The error rate of the apparatus according to the invention is ascertained by comparative measurement, with the apparatus and simultaneously a reference method, such as a light barrier in conjunction with an automated digital image processing and pattern recognition system, being employed. Any false-positive or false-negative detection of a stationary or travelling vehicle and any false classification of a true-positive detection is rated as an error. Vehicles sensed by the apparatus are classified into eight or nine categories, for example: car, car with trailer, truck, truck with trailer, van, bus, articulated lorry, motorcycle and miscellaneous. The error rate denotes the ratio of all of the errors to all detection processes. A false-positive detection can occur when the apparatus incorrectly indicates an entry process for a parking space in a parking lot even though a vehicle movement is taking place just on an adjacent parking space. In the case of a false-negative detection, a stationary or travelling vehicle is incorrectly not recorded.
One of the causes of errors in the magnetic-field-based detection of vehicles is that the known apparatuses involve the magnetic field not being measured separately according to the spatial directions or vectorial fractions. The reason for this is primarily that when the known apparatuses are installed the magnetometers are not fitted in a precisely defined spatial orientation. Even though precise alignment of the magnetometers does not face any fundamental obstacles, the work involvement associated therewith is considerable. A further problem is that ground settlement, thermal effects or road damage in the course of a few months to several years means that the orientation of the magnetometers can change and the original alignment settings are lost.
In Northern latitudes, such as Northern and Central Europe, the vertical component of the Earth's magnetic field is greater than the horizontal component. On the geomagnetic equator, which runs through the North of Brazil and Nigeria, inter alia, the vertical component of the Earth's magnetic field is infinitesimally small, on the other hand. Within the context of the present invention, the terms “vertical component” and “horizontal component” have their usual meaning and relate to directions that point radially away from the Earth's center at the respective geographical location, or are situated in a plane that is oriented parallel to the ground or perpendicular to the vertical component. The horizontal component of the Earth's magnetic field is directed outside the globe, apart from slight geographical deviations from the geomagnetic North Pole to the geomagnetic South Pole. The metal parts of a vehicle shield the Earth's magnetic field and/or distort the Earth's magnetic field. Frequently, the Earth's magnetic field is altered such that the magnetic field strength or the magnetic field lines are attenuated in directionally dependent fashion in the surroundings of the vehicle. When a vehicle, such as a car or a truck, is moved past a physically fixed magnetometer for measuring the Earth's magnetic field, the magnetometer records a temporally varying, transient or sudden decrease in the magnetic field strength when the vehicle passes. As soon as the vehicle has passed the magnetometer completely and is moving away, the magnetic field strength and hence the output signal from the magnetometer rise back to the previous value or level. Some vehicle components contain magnetizable materials, such as soft iron, which are tracked or focus the field lines of the Earth's magnetic field. When such a vehicle component is driven past a magnetometer at a distance of up to a few meters, this magnetometer records a temporally varying, transient or sudden increase in the magnetic field strength. The speed at which the attenuation or gain in the Earth's magnetic field takes place at the location of the magnetometer is dependent firstly on the speed of the vehicle and secondly on the angle between the direction of movement of the vehicle and the direction of the magnetic field lines. For the vertical component of the Earth's magnetic field, this angle is always 90°. For the horizontal component, on the other hand, this angle can vary between zero and 180°. Depending on the angle at which the vehicle moves in relation to the horizontal component of the Earth's magnetic field, the horizontal component of the Earth's magnetic field is attenuated or amplified to a greater or lesser extent in the surroundings of the vehicle. This means that the time-based derivation or the time-based differential of the horizontal component of the Earth's magnetic field is dependent on the vehicle speed and the angle between the direction of movement of the vehicle and the horizontal component of the Earth's magnetic field.
The above explanations reveal that measurement of the Earth's magnetic field in a manner separated according to vectorial components, particularly separation into the vertical component and the horizontal component of the Earth's magnetic field, is advantageous and allows the measurement accuracy and detection sensitivity to be increased.